Conventionally known as a heating device is a fixing device for heating a sheet using a plate-shaped heat generating member. This fixing device is configured such that the surfaces of the plate-shaped heat generating member and a pressure roller face each other. This fixing device is configured such that the plate-shaped heat generating member is in contact with the inner surface of an endless belt and the opposite surface of the endless belt is in contact with a first surface of a sheet, thereby heating the sheet via the endless belt. This fixing device is also configured such that the pressure roller and the second surface of the sheet are in contact with each other, allowing the plate-shaped heat generating member and the pressure roller to apply pressure thereto. This allows the fixing device to fix a toner image transferred to the sheet onto the sheet.
Some fixing devices employ a ceramic heater as a heat generating member.
A sheet width direction refers to a direction orthogonal to a sheet conveyance direction. In a conventional technique, a plurality of heat generating parts are formed on a ceramic substrate and arranged in a sheet width direction. The conventional technique prevents unnecessary heat generation by controlling the energization of the heat generating parts depending on the size of a sheet to be subjected to a fixing treatment.
According to a first conventional technique, a number of heat generating parts of the same width are formed and arranged in the sheet width direction. The first conventional technique collectively performs ON/OFF control, on the basis of the detected temperature of the endless belt, to the output from a group of heat generating parts located at a location corresponding to the size of an image to be formed.
In the first conventional technique, since the temperature of the endless belt is detected to collectively adjust the output from the group of heat generating parts, it is not possible to adjust the temperatures of a plurality of regions in the sheet width direction. Thus, in the first conventional technique, it is not possible to perform such control that the center portion in the sheet width direction provides high output and end portions provide low output because the sheet does not pass therethrough, while monitoring the detected temperatures of the respective center portion and end portions. That is, the first conventional technique has a possibility of making an improvement in that finer control can be performed to the amount of heat generation in the sheet width direction.
On the other hand, in a second conventional technique, a number of heat generating parts of the same width are formed and arranged in the sheet width direction. In the second conventional technique, on the rear surface of the ceramic substrate on which no heat generating parts are formed, a thermistor is brought into contact with a region across two heat generating parts in plan view that are located at the center in the sheet width direction and detects temperatures. In the second conventional technique, the output of the group of heat generating parts located at a location corresponding to the size of a sheet is collectively adjusted on the basis of the aforementioned detected temperature.
The second conventional technique detects one point on the ceramic substrate and collectively adjusts the output of the group of heat generating parts. Thus, in the second conventional technique, it is not possible to perform such control that the group of heat generating parts at the center portion in the sheet width direction provides high output and the groups of heat generating parts at end portions provide low output, while monitoring the detected temperature of the group of heat generating parts at the respective portions. That is, the second conventional technique has a possibility of making an improvement in that finer control can be performed to the amount of heat generation in the sheet width direction.
In a third conventional technique, the widths of heat generating parts are different depending on the respective locations. Of the heat generating parts arranged side by side in the sheet width direction, the width of the first heat generating part at the center corresponds to the width of A5 size. The total value of the width of a pair of second heat generating parts located on both outer sides of the first heat generating part and the width of the first heat generating part is set to be equal to the width of A4 size. The total value of the width of a pair of third heat generating parts located on both outer sides of the second heat generating parts and the width of the first and second heat generating parts is set to be equal to the width of A4-R size.
In the third conventional technique, on the rear surface of the ceramic substrate on which no heat generating parts are formed, a thermistor is brought into contact with a location which overlaps each of the first to third heat generating parts in plan view and detects the temperature of each location. In the third conventional technique, the first to third heat generating parts provide output depending on the sheet size and the output is adjusted on the basis of the each detected temperature mentioned above.
In the third conventional technique, for the sheet of A5 size, the first heat generating part provides high output, while for the sheet of A4 size, the first and second heat generating parts (three heat generating regions) provide high output. In the third conventional technique, the heat generating regions on the ceramic substrate in the sheet width direction are roughly divided. However, the heat generating parts are one continuous resistive member, and thus no consideration is given to a configuration like a plurality of heat generating groups with gaps therebetween.